The present invention relates generally to the field of plasticating components, such as screws and barrels, used for extruding plastic. In particular, the present invention is directed to a structure for a more abrasion-resistant and corrosion-resistant plasticating barrel, and a technique for manufacturing the lining of the improved barrel.
Extruders and tubers (rubber extruders) have been in use at least since the beginning of the twentieth century. With the advent of plastics, the demand for such extruders has become greater and the processing conditions have become more severe. Originally such devices were essentially a simple screw rotating inside a single-material barrel without a lining. This is no longer the case due to the newer and more difficult to process materials.
Both of these components are subject to wear from metal-to- metal contact, and from abrasive and corrosive fillers in the plastic or rubber compounds. The original barrels had an internal surface that was nitrided to give improved abrasive wear resistance. In the later 1950""s bimetallic barrels were developed using a centrifugal casting process, as briefly described in the Spirex publication, entitled Plasticating Components Technology, 1997, incorporated herein by reference. Also, such improved barrels were adapted for use with injection molding machines, in addition to conventional extruders.
Centrifugal casting of plasticating barrels is a process used to line the internal surface of a barrel with an abrasion and/or corrosion resistant liner that is different from the barrel backing material or substrate. The process involves installing a lining material, such as a powder, inside the heavy-walled barrel cylinder at room temperature. The ends of the barrel are capped (usually welded) and the barrel and unmelted powder are placed in a casting oven. The barrel is then rotated and heated until the liner material metals are melted uniformly distributed on the internal surface of the barrel. Early liner materials were iron/boron materials that created some metal carbides and were very much more wear resistant that the nitrited barrels. In 1968 improved liners became more abrasion resistant by the addition of very small, discrete metal carbides particles like tungsten carbide and equivalent materials.
Most rotational casting ovens are gas heated but some are induction heated. In either case, the inside of the barrel must be heated to a point where the liner powder melts, but the thick-walled barrel material or substrate does not melt. After melting is accomplished the barrel is slowly cooled so that stresses are not induced, and so that the liner material does not crack. After cooling, the barrel is honed, straightened and machined to it""s final dimensions. Often this requires installation of a high-pressure sleeve at the discharge end of the barrel.
There are a number of disadvantages to this technique. The gas fired or induction furnaces with rotating equipment are very expensive, and require extensive maintenance. This includes periodic and prolonged shutdowns to reline the refractory surfaces of the oven. Further, even when the furnaces are functioning properly, set up for the coating of each barrel is an awkward and time, consuming process.
Also, the process of centrifugal coating requires that the liner material or material matrix melt at a lower temperature than the backing or substrate material. This creates severe limitations on the liner materials than can be used. As a result, abrasion-resistant and corrosion-resistant materials are limited to formulas that melt at a lower temperature than the barrel substrate. In many cases the optimum barrel substrate and under materials cannot be used for the materials to be handled.
There is also the requirement of raising the backing or substrate material to a temperature close to the melting point of the substrate material followed by a slow cooling to anneal the backing material. This lowers the strength of the annealed backing material. Unfortunately, very high strengths are now required because such barrels can be subject to internal pressures of 40,000 psi or higher, and temperatures up to 700 deg. F. These conditions require the installation of a high pressure sleeve at considerable expense. Some newer, higher priced alloys can reduce this effect somewhat by reducing the loss of strength. However, greater expense is incurred.
During the rotational casting process the heavier metal carbide particles tend to be thrown outward by centrifugal force. This moves these particles away from the inside surface where they are needed for abrasion resistance. As a result, the resulting lining is far more susceptible to wear caused by abrasion than if the metal carbide particles are properly located on the inner surface of the lining or evenly distributed throughout the lining or cladding.
The high barrel temperatures that are reached during casting make it difficult to maintain the straightness which is critical to the plastic processing operation. Straightening of the barrel cannot be done by conventional straightening presses because reverse bending cracks the relatively brittle liner. The rotational casting process requires a long time to heat up the liner and barrel substrate. Additional time is required for slow cooling after the lining operation. This causes added expense in labor and electrical costs.
Because the lining process can only be successful in a very narrow range of time and temperatures, often the results are not satisfactory. High temperatures and long time periods spent at these temperatures cause dilution by migration of the substrate material into the barrel lining material. This causes poor hardness and poor abrasion resistance. Also substrate migration of the base iron material can cause poor corrosion resistance in certain applications. Extended periods at high temperatures also cause the metal carbide particles coating the liner inner surface to melt into solution in the matrix matter (constituting the liner) rendering them useless.
When temperatures are too low and the time periods at properly elevated temperatures are too short, an inadequate metallic bond can result. Such an inadequate metallic bond means that the liner may become separated from the barrel substrate or backing material. This condition could render the entire barrel useless. Further, in some cases portions of the liner may come dislodged, corrupting the molten plastic and/or fouling the screw pushing the molten plastic through the barrel. In either case, the barrel is subject to catastrophic failure, and the plastic processed therein ruined.
A totally different method to produce barrel liners is constituted by laser welding or cladding. Laser cladding is laser welding of a different surface onto a base or substrate metal. This new process diminishes or eliminates all of the disadvantages listed above.
The more conventional MIG or TIG welding of the inside diameter (ID) of barrels can be done, but it is more difficult to get into smaller diameter barrels. The zone affected by heat is much greater, and the welded surface is poorer, causing much greater expense in finishing compared to the xe2x80x9cnear-net shapexe2x80x9d of laser-welding.
Laser welding of the ID of barrels involves the depositing of the liner material prior to welding in the form of paste or a separate liner tube, or during welding with a powder or continuous wire. The laser welder usually includes a laser beam delivered from a remote source via fiber optics and optical systems, or by direct laser beams.
This technique has a number of advantages. For example, devices have been made that will allow laser welding into diameters as small as xc2xe inch. Laser cladding also has a very shallow heat-affected depth which gives much less dilution of the liner material into the barrel substrate. This technique also creates much less stress in the substrate, reducing the tendency to bend or warp.
Laser cladding is a relatively robust process that allows a wide latitude of materials to be used, including materials that melt at higher temperatures than the barrel substrate. This can lead to improved matrix materials and improved ceramic or carbide materials as anti-abrasive coatings on the liner. Discrete abrasion resistant carbide or ceramic particles do not migrate toward the substrate as in rotational casting. This leaves them evenly distributed where they are needed.
The substrate does not necessarily need to be preheated prior to welding, thus reducing production time and expense. Heat imparted by the laser welding process is much reduced and can be removed during welding by internal and external methods. This means that a long cooling down time can be eliminated. As a result, the process is less time-consuming than centrifugal casting.
Laser welding is an actual welding process with a metallurgical bond rather than a brazing process where the liner melts at a lower temperature than the substrate as in rotational casting. The laser cladding equipment is generally lower cost than gas-fired or induction furnaces.
Several devices to laser clad the inside of pipes have been invented and commercialized. These include EPRI Patent Nos. 5,653,897 and 5,656,185 and IHI Patent No. 5,426,278. Also included are U.S. Pat. Nos. 5,496,422; 5,196,272; and, 5,387,292. All of the aforementioned patents are incorporated herein by reference to facilitate a better understanding of the present invention. These devices are designed to repair damaged or corroded heat exchanger tubes in power generation plants. These systems are designed to make short, localized repairs in relatively long, fixed pipes that cannot rotate. Consequently, each of these systems uses a rotating laser head for welding. The systems described in the aforementioned patents include the insertion of a cladding or inlay material by wire, powder, paste, and thin wall tube. The paste and the tubes are already in place before laser cladding. In the case of the EPRI patent, a coiled wire is placed inside the pipe directly above the repair area in order to have it easily accessible and easy to feed as the cladding proceeds. This method is limited to short longitudinal lengths of welds as is generally required in boiler repair. Powder is difficult to introduce in the horizontal position because, without gravity assist, it tends to clog and interrupts cladding. Drawings of these various welding devices are shown in the patents.
For prolonged or full length cladding of 20:1 L/D or longer pipes the head and especially the reflecting mirrors must be cooled. This can be done by a cooling fluid such as air or water. The EPRI patent does not have such cooling except for the bearings that are required to rotate the head inside the pipe. The IHI device allows cooling (by air) coming from the direction of the laser source.
All of these devices must have all auxiliary services introduced from the laser head end of the tube because access from the opposite end is not available, and cannot be coordinated with the activity provided from the laser end. These auxiliary services can include fiber optical viewer, wire/powder feeds, cooling media, optics (lenses) and focusing devices.
The devices disclosed in the subject patents weld on constantly changing surfaces. This tends to give a non-uniform and less smooth surface due to the influence of gravity. If the cladding is done with the pipe vertical, the melt pool tends to not be flattened and can have exaggerated rings or other distortions in the surface. In any case, there is no natural tendency to flatten and smooth the surface in a uniform manner.
Also materials currently used in conventional laser welding processes are used primarily for corrosion resistance. This limited application of the conventional technology is adequate since the boiler tubes (in which conventional laser welding occurs) are not exposed to the abrasion of the types of materials handled by plasticating barrels.
There is also a need to make such devices smaller than the standard commercial sizes now available. In particular, barrel I.D.""s as small as 14 mm (0.551 inch) are used for plasticating barrels. Thus, appropriate welding devices are necessary to clad or line the interior of the plasticating barrels. Conventional rotating welding devices operate entirely from one end of the tube being lined or welded. Consequently, size reduction for such welding devices is severely limited. This is particularly true since the welding head must include all auxiliary services, as well as the bearings. This entire structure is fed into the tube to be welded from only one side of the tube. As a result, size reduction of the overall welding apparatus is very problematical, and cannot accommodate some smaller sizes used for plasticating barrels.
Accordingly, there is a need for a system capable of addressing the smaller sizes of plasticating barrels, and to provide smooth, uniform inner linings to such plasticating barrels. Of necessity, such a system will have to be flexible, and capable of using a number of different techniques to produce an optimum product at reasonable costs.
It is one object of the present invention to provide a system for lining tubes or any interior surface that overcomes the drawbacks of the conventional art.
It is another object of the present invention to provide a system for inside diameters (I.D.) that is sufficiently flexible so that a wide variety of tube sizes and cladding materials can be accommodated.
It is a further object of the present invention to provide a system for lining inside diameters of tubes capable of accommodating smaller I.D. sizes than is currently possible with conventional techniques and systems.
It is an additional object of the present invention to provide a system for lining inside diameters of tubes in which a more uniform lining can be achieved than is possible with existing welding techniques.
It is again a further object of the present invention to provide a system for lining of I.D.""s in which a smoothing agent operates in a uniform manner on each of the welds constituting the cladding of the plasticating barrel.
It is still another object of the present invention to provide a system for laser-cladding the interior of a metal tube wherein the system is relatively simple to set up compared to conventional systems.
It is again a further object of the present invention to provide a process for laser-cladding the interior of a metal tube, requiring reduced operating time.
It is still another object of the present invention to provide a system for quickly and easily repairing the lining of metal tubes at lower expenditures than those incurred with conventional systems.
It is again a further object of the present invention to provide a system for lining the interior of the metal tube by laser-welding wherein the conventional necessity of a rotating laser head is avoided.
It is yet another object of the present invention to provide a system for lining a metal tube using laser welding in which movement of the welding head is required only along the axis of the tube being welded.
It is again a further object of the present invention to line a metal tube using laser welding to which an anti-abrasive material is added so that the anti-abrasive material remains uniformly distributed in the laser-welded cladding.
It is yet another object of the present invention to provide a smoother, pre-machined weld that is obtainable from conventional welding techniques, in particular, MIG and TIG methods.
It is yet another object of the present invention to provide a method of uniformly precoating an accurate amount of material on an interior surface to facilitate welding operations thereon.
It is still a further object of the present invention to provide a system of precisely placing a uniform pattern of anti-abrasive material in a weld melt without melting or otherwise degrading the anti-abrasive material.
It is yet another object of the present invention to facilitate faster, pre-weld set up of plasticating barrels.
These and other goals and objects of the present invention are achieved by a plasticating barrel adapted for use with extruded molten plastic. The barrel includes a substrate composed of a first metallic material and a liner composed of a second metallic material. The liner is fabricated by laser welding to achieve a substantially uniform cladding over the entire diameter of the barrel. The liner is formed to have an inside diameter of less than 15 mm as finished by laser welding.
In another embodiment of the present invention, a plasticating barrel is adapted for use in extruding molten plastic, and includes a substrate of a first metallic material and a liner of a second metallic material. The liner is formed by laser welding of the second metallic matrix material to clad the interior of the barrel. An anti-abrasive layer is formed of a third material composition and is uniformly arranged in unmelted form throughout the metallic matrix.
Another embodiment is constituted by a system for laser-welding a lining to the interior of a metal tube. The system includes a laser welding head arranged to enter the metal tube from a first end of the metal tube. The system also includes auxiliary equipment arranged to enter the metal tube from the second end of that metal tube.
In yet an additional embodiment of the present invention, a system for lining a metal tube by laser-weld cladding is provided to include a laser aiming optic head. Also included is a device for holding the metal tube in a horizontal position and rotating the tube about a horizontal axis. Another device is used to position the laser aiming optic head so that welding always takes place in a single direction. A controller coordinates the rotational movement of the metal tube and the operation of the laser aiming optic head.
It is still a further embodiment of the present invention a system is provided for lining an interior surface. The system includes a laser aiming optics head and a mechanism for welding/cladding on the bottom surfaces thereby applying gravity to create a series of smooth uniform welds that constitute the lining of the metal tube.
Yet another embodiment of the present invention includes a system for lining a metal tube by laser-welding. The system includes a laser source arranged to emit laser light into a first end of the metal tube. The system also includes a laser aiming optics head arranged to deflect light from the laser source, and arranged to enter the metal tube at a second end.
Yet an additional aspect of the present invention includes a system for lining a metal tube by laser welding. The system includes a laser aiming optics head arranged to deflect laser light entering the metal tube at the first end of the metal tube. The laser aiming optics head is rotateably mounted an arranged to enter the metal tube at a second end of the metal tube.
Another aspect of the present invention is manifested by a method of lining a metal tube by laser welding. The process includes the step of arranging the metal tube in a horizontal position. Next a laser aiming optics head is placed in the metal tube. Then a series of laser welds are carried out in a single direction while rotating the metal tube and moving the laser aiming optics head in a single direction along the metal tube.
Again another aspect of the present invention is a method of lining a metal tube by laser-welding, where the method includes the steps of placing a laser aiming optics head in the metal tube at a first end of that tube. Then, laser light is transmitted from a second end of the metal tube to be deflected by the laser aiming optics head to effect laser welding in the metal tube.
Yet an additional aspect of the present invention is a method of cladding an interior surface by laser welding. The method includes the first step of arranging a laser aiming optics head to operate in a predetermined pattern along the interior surface to create a series of weld melts. Each of the weld melts has a warmer heating portion and a cooler trailing portion. Then, an anti-abrasive material is added to the trailing portion of each of the weld melts. As a result the anti-abrasive material is undegraded and uniformly distributed through each of the weld melts.
An additional aspect of the present invention is manifested by a plasticating barrel adapted for use of extruding molten plastic. The barrel is of a first metallic material, and is provided with a laser clad lining of a second metallic material. The second material is a mixture of nickel and chromium.
Still an additional aspect of the present invention is a method of lining a tube including a first step of inserting a slurry into the tube. A slurry includes liner material. Then the tube is centrifugally cast to form a uniform hard coating of the slurry to adhere to the inside diameter of the tube. Then laser welding is carried out on the coating to form the finished metallic lining.